Composition for controlling scale in black liquor evaporators

ABSTRACT

An anionic/cationic polymer mixture comprising: 
     (a) a polyanion selected from polyacrylic acid, polymethacrylic acid, and polymaleic anhydride, each optionally copolymerized with each other, or optionally copolymerized with acrylamide up to a molar unit ratio of 1:1, provided that the total polyanionic weight average molecular weight is from about 1 to 5 thousand, and 
     (b) the polycation poly(dimethyldiallylammonium chloride) having a weight average molecular weight of from about 25 thousand to 1 million, 
     wherein the weight ratio of polyanion (a) to polycation (b) is from 1:2 to 1:8, has been found to give excellent inhibition of the formation, deposition and adherence of various alkali metal and alkaline earth metal scales, e.g., sodium sulfate, sodium carbonate, and calcium carbonate scales in the stringent conditions which characterize black liquor heaters, evaporators, etc. in paper mills, particularly in association with Kamyr and similar wood chip digesters, characterized by elevated pH&#39;s of 12 and higher, high dissolved organic and inorganic solids concentrations, and elevated temperatures and pressures.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.708,522, filed May 31, 1991, now abandoned.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to compositions and methods for inhibitingthe formation, deposition and adherence of alkali metal and alkalineearth metal scale deposits, e.g., sodium sulfate (Na₂ SO₄), sodiumcarbonate (Na₂ CO₃), and calcium carbonate (CaCO₃) scale deposits, onmetallic surfaces of heaters, evaporators and other heat exchangersurfaces used for the processing of black liquor in the paper makingprocess, particularly in association with Kamyr and similar wood chipdigesters.

Origin of Black Liquor

In the production of kraft paper by the sulfate process, bark andchipped wood are treated with an alkaline aqueous liquid to removecertain organic contaminants from the wood, of which lignin is the chiefcomponent. Typically, commercially, the chips are cooked in a 10 percentsolution of sodium hydroxide which contains about 20 mole percent ofsodium sulfide. This reaction is usually conducted at temperatures ofapproximately 160° to 180° C. for a period of time ranging between 1 and3 hours.

The resultant organic residues are removed from the chips by washing,which wash water contains dissolved lignin, emulsified soaps, endogenoussurfactants, other organic ingredients, and substantial amounts ofinorganic salts and bases, including calcium carbonate. This wash wateris referred to as black liquor.

Black Liquor and Scale Formation

The black liquor will show variations in composition from mill to mill;however, in most instances, inorganic carbonates, sulfides, sulfites,sulfates and silica are present, as well as organic sulfur compounds.The makeup of a typical black liquor is set out further below. While themakeup of the black liquor may vary from time to time even where thesame type of pulp is being produced, and will clearly vary wheredifferent types of pulp are involved, the same stringent conditions areconfronted in the processing of black liquor which have heretoforerendered practical treatment of scale deposits ineffective.

These stringent conditions are characterized by elevated pH's of 12 andhigher, high dissolved organic and inorganic solids concentrations, andelevated temperatures and pressures. Black liquor is processed in twoways which bring it into contact with heat exchange surfaces leading tothe formation of adherent scale deposits which are speculated to be inaccordance with well known mechanisms described in detail further below:evaporative recovery and the wood chip digestion process itself. Both ofthese will now be described in more detail.

Black Liquor Evaporative Recovery

It is common practice in large mills to recover the inorganic componentsof the liquor and to use the organic portion as fuel. As produced, theblack liquor will usually contain about 12 percent by weight of solidmaterial. Before the liquor can be used as fuel and the inorganiccomponents recovered, it is necessary that the material be concentrated,usually to a solids content of about 45 percent by weight or higher. Theconcentrating of the black liquor is usually conducted inmultiple-effect evaporators. These evaporators are ordinarily operatedby employing the steam produced in the highest pressure evaporator toheat the next highest pressure evaporator. The flow of liquor iscounter-current to the flow of pressure and steam. In evaporatorscontaining black liquor of the lowest solids concentration, the steamused usually produces a vacuum in the system.

A common problem which arises with use of a black liquor multiple-effectevaporator is formation of substantial amounts of deposits which tend tostick to the interior walls or tubes of the evaporator units and remainin a tightly adherent state. The problem of preventing deposition ofmaterial upon the metal surfaces of the various units of amultiple-effect evaporator is particularly difficult to overcome in thatthe black liquor has a tendency to deposit out both inorganic andorganic materials. The organic substances then act as a type of binderfor the inorganic materials, with the entire mass becoming a tightlyadherent mass clinging to the metal surface walls of the evaporators. Itis believed that this phenomenon accounts for the reason that many priorart deposit inhibitors show little or no activity in inhibitingdeposition of materials in a black liquor evaporator system.

A buildup of deposit formation results in a situation of measurablydecreased overall efficiency of evaporation. For example, the depositformation tends to materially decrease heat transfer, requiring anincreased heat input to accomplish desired evaporation. Likewise, theproblem of deposition leads to more frequent boil-outs with hot water oracids and a substantial increase in down-time. Deposit formation canoccur in any of the effects of the multiple effect evaporator. There areusually 5 to 6 effects in each evaporator set, each effect containing aplurality of long-tube vertical units.

Black Liquor Digester Processes

Another area of the paper making process in which black liquorprocessing brings the black liquor into contact with heat exchangesurfaces and leads to adherent scale deposits, and therefore with whichthe compositions and methods of the present invention are especiallyusefull, is the Kamyr and similar wood chip digesters, which have beenin commercial use for several decades. Processes using these digestersinvolve heating of the black liquor and its subsequent use as part ofthe digestion of the wood chips to form pulp. Thus, adherent scaledeposits tend to form not only on the surfaces of the black liquorheaters, but on the surfaces of the Kamyr and other digestersthemselves.

In the vertical downflow type of continuous Kamyr digester, wood chipsare fed at low pressure to a horizontal steaming vessel, where a slowlyturning screw carries the chips and exposes them to flash steam whichpreheats the chips and drives off air and other noncondensibles. Thechips are then carried by a cooking liquor to a cylindricalseparator/digester where a perforated plate allows liquor to flow to thesurrounding collection ring and be returned to the feeder, while thechips are pushed downwardly by a rotating helical screw. A hydraulicpressure of 165 psig is maintained on the liquid column by regulatingthe flows of white and black liquor to the digester.

The chips flow downward uniformly as a mass through the various zones ofthe digester: impregnation, heating, cooking and washing. The durationof the alkali impregnation zone is about 45 min at temperatures from105°-130° C. The temperature is raised in two steps in the heating zoneusing forced circulation of the black liquor through external heatexchangers. The pulping reactions are then completed in the cookingzone, and stopped by displacement of the hot residual liquor with dilute"wash" liquor from below. The hot liquor is extracted through screens atthe periphery into a flash tank that supplies steam to the steamingvessel.

The wash liquor moves in the opposite direction to the downward chipflow, and thus is injected at the bottom of the digestion vessel andmoves upward, allowing it to provide "diffusion washing". The overallheight of the digester is sufficient, taking into account the distancebetween the wash water inlet and the extraction screens and rate ofmovement of the liquor, to allow at least 1.5 hrs for the residualliquor to diffuse out of the chips. Pulp is continuously removed fromthe bottom of the digester, which is maintained at about 200 psig., toan atmospheric tank.

Modifications of the basic Kamyr hydraulic digester process describedabove which relate to the potential formation of adherent scaledeposits, have included development of the vapor phase digester forsulfite and prehydrolyzed kraft production, which utilizes an inclinedseparator, and the development of a two-vessel system with a separateimpregnation vessel and vapor phase digester in which the outsideinclined separator has been replaced with an inverted internalseparator.

It can be seen from the above description that there are a number oflocations in the equipment for carrying out the wood chip digestionprocess at which there is potential for the formation of troublesomeadherent scale deposits derived from black liquor. It is to theinhibition of all such deposits that the compositions and methods of thepresent invention are directed.

Adherent Scale Formation

The description above relates to the very significant and complexproblems which are encountered when black liquor is contacted with heatexchange surfaces during various phases of the paper making process. Tosome extent, although not fully understood, the basic mechanisms ofscale formation which are fairly well known in such traditional areas asboilers and cooling towers, doubtless play some role in scale formationfrom black liquor in paper making. Thus, in order to provide a betterunderstanding of the present invention, those basic mechanisms will nowbe briefly described.

As already mentioned, black liquor will contain a number of dissolvedsalts, and the alkali metal cation sodium is usually prevalent, whilethe alkaline earth metal cation calcium is also present, as are theanions sulfate and carbonate. The combination product of sodium andcalcium cations and sulfate and carbonate anions will precipitate fromthe black liquor in which they are carried to form scale deposits whenthe concentration of the anion and cation comprising the reactionproduct, e.g., sodium sulfate, sodium carbonate, and calcium carbonate,exceeds the solubility of the reaction product itself. Thus, when theconcentrations of sodium and calcium ions and sulfate and carbonate ionsexceed the solubility of the sodium sulfate, sodium carbonate, andcalcium carbonate reaction products, a solid phase of sodium sulfate,sodium carbonate, and calcium carbonate will form as a precipitate.Precipitation of the reaction product will continue until the solubilityproduct concentrations of the constituent ions are no longer exceeded.

Numerous factors may be responsible for producing a condition ofsupersaturation for any given reaction product. Among such factors arechanges in the pH of the water system, evaporation of the water phase,rate of heat transfer, amount of dissolved solids, and changes in thetemperature or pressure of the system. As already discussed, all ofthese factors are present in the stringent conditions which characterizeblack liquor processing in paper making.

The mechanism of scale formation is apparently one of crystallization ofscale-forming salts from a solution which is locally supersaturated inthe region adjacent the heating surface of the system. The thin viscousfilm of water in this region tends to become more concentrated than theremainder of the solution outside this region. As a result, thesolubility of the scale-forming sodium sulfate, sodium carbonate, andcalcium carbonate salt reaction products is first exceeded in this thinfilm, and crystallization of the respective scales results directly onthe heating or heat exchange surface.

In addition to this, a probable source of scale in black liquorprocessing in paper making is the concentration of solids dissolved inthe water of the black liquor not only by leaching of solids from thedigesting wood chips, but also by repeated evaporation of portions ofthe water phase, as, e.g., when portions of the black liquor are flashevaporated to provide steam. Moreover, alkalinity from the alkalidigesting solution as well as from dissolved solids from the wood chips,results in an increasing alkalinity of the black liquor, often reachingpH's of 12-13 and even higher. Conventional scale inhibitingcompositions typically fail in systems having such severe conditions.

As already adverted to, the formation of sodium sulfate, sodiumcarbonate, calcium carbonate and other scale deposits poses a seriousproblem in a number of regards. The sodium sulfate, sodium carbonate,calcium carbonate and other scales which are formed posses a low degreeof heat conductivity. Thus, e.g., a sodium sulfate or calcium carbonatescale deposit is essentially an insulating layer imposed across the pathof heat travel from whatever source to the black liquor being processedin the system. Increased input of heat to compensate for this lossresults in overheating of the metal surface of the heat exchanger andconsequent tube failures, for example. In addition to this problem,scale formation facilitates corrosive processes, and a substantial scaledeposit will interfere materially with fluid flow. Consequently, scaleis an expensive problem in the paper making process, causing delays andshutdowns for cleaning and removal.

Although the present invention is directed primarily to preventing orinhibiting the deposition of sodium sulfate, sodium carbonate, andcalcium carbonate scales, the most prevalent types of scale deposits, itis also applicable to inhibiting the deposition of other types of alkalimetal and alkaline earth metal scales. For example, black liquorcontains alkali metal cations such as sodium and potassium, and alkalineearth metal cations, such as calcium, magnesium, etc., and severalanions such as bicarbonate, carbonate, sulfate, oxalate, phosphate,silicate, fluoride, etc. When combinations of these anions and cationsare present in concentrations which exceed the solubility of theirreaction products, precipitates form until their product solubilityconcentrations are no longer exceeded. These precipitates are alkalimetal and alkaline earth metal scales. Thus, by alkali metal andalkaline earth metal scales is meant scales including but not limited tosodium sulfate, sodium carbonate, calcium carbonate, magnesiumcarbonate, calcium phosphate, and calcium sulfate. These scales formfrequently in the tubes of heat exchangers and on other heat exchangesurfaces, as well as on various surfaces of digester equipment incontact with black liquor being processed during paper making.

The anionic/cationic polymer mixtures of the present invention are usedin the same range of amounts as threshold inhibitors in the scaleinhibition method of the present invention, rather than as sequesteringor chelating agents, even though the compositions of the presentinvention appear to have dispersant properties as well as significantlyreduce the adherency of any scale deposit which is formed, facilitatingits easy removal.

Heretofore, in the art of scale inhibition as it related to thetraditional areas of boilers and cooling towers, it was recognized thatscale-forming compounds could be prevented from precipitating byinactivating their cations with chelating or sequestering agents, sothat the solubility of their reaction products was not exceeded.Generally, this required many times as much chelating or sequesteringagent as cation, since chelation is a stoichiometric reaction, and theseamounts were not always desirable or economical. However, severaldecades ago, it was discovered that certain inorganic polyphosphateswould prevent such precipitation when added in amounts far less than theconcentrations needed for sequestering or chelating. Nevertheless, evensuch agents as these have not been found to be particularly effective incontrolling scale deposits on equipment used for black liquorprocessing.

When a precipitation inhibitor is present in a potentially scale-formingsystem at a markedly lower concentration than that required forsequestering the scale-forming cation (stoichiometric), it is said to bepresent in "threshold" amounts. See, for example, Hatch and Rice,Indust. Eng. Chem., 31, 51-53 (1939); Reitemeier and Buehrer, J. Phys.Chem., 44 (5), 535-536 (1940); Fink and Richardson U.S. Pat. No.2,358,222; and Hatch, U.S. Pat. No. 2,539,305.

Generally, sequestering takes place at a weight ratio of sequestrationcompounds to scale-forming cation components of greater than about 10:1,depending on the anion components in the water. Threshold inhibitiongenerally takes place at a weight ratio of threshold active compounds toscale-forming cation components of less than about 0.5:1.0.

Similarly, anionic and cationic polymers can be used as dispersants inaccordance with methods known in the art of scale inhibition, but thedosage levels necessary to achieve dispersion are in the range of0.5-10% by weight of the system being treated, which is many orders ofmagnitude higher that the dosage levels used for the compositions of thepresent invention. Thus, it is a unique aspect of the present inventionthat it is possible to achieve essentially non-adherent scale on blackliquor processing equipment using only threshold inhibitor dosage levelsof the compositions of the present invention.

Recently in the art of scale inhibition, attention has been focused oncontrolling scaling under severe conditions, where conventionaltreatments such as those described above do not provide complete scalecontrol, and these are thought to more closely duplicate the stringentconditions found in processing of black liquor. Current technology intraditional scale control, e.g., in cooling towers, can be used toinhibit CaCO₃ scale up to 100 to 120 times calcite saturation, i.e., awater containing Ca²⁺ and CO₃ ²⁻ present at 100 times (100×) theirsolubility limit. Severity of the scaling tendency of a water sample ismeasured using the saturation index, which may be derived in accordancewith the following equation: ##EQU1## where SI is the saturation indexfor calcium carbonate, (Ca²⁺) is the concentration of free calcium ions,(CO₃ ²⁻) is the concentration of free carbonate ions, and ^(K) spCaCO₃is the conditional solubility product constant for CaCO₃. All of thequantities on the right side of the above equation are adjusted for pH,temperature and ionic strength.

One problem which will usually be encountered with scale inhibitingcompositions known in the traditional art of scale inhibition is theircalcium tolerance. Calcium tolerance is a measure of a chemicalcompound's ability to remain soluble in the presence of calcium ions(Ca²⁺). One of the parameters of scale control under severe conditionsis pH. As pH increases, calcium tolerance decreases rapidly fortraditional CaCO₃ threshold inhibitors, e.g., HEDP and AMP. Theseinhibitors precipitate with calcium at alkaline pH's, rendering themuseless as threshold scale inhibitors. Since the high pH and highcalcium ion concentration which characterize severe conditions in thetraditional art of scale inhibition may also characterize some of thestringent conditions encountered in the processing of black liquor, itis conjectured that this factor may be responsible for the failure oftraditional threshold inhibitors used heretofore to provide effectivescale control on equipment used to process black liquor.

BRIEF DESCRIPTION OF THE PRIOR ART

In the traditional scale inhibition art relating to boilers, coolingtowers, etc., early efforts to reduce scale formation employed compoundssuch as tannins, modified lignins, algins, and other similar materials.Chelating or sequestering agents were also employed to preventprecipitation or crystallization of scale-forming calcium carbonate.Another type of agent which has been actively explored heretofore as acalcium carbonate scale inhibiting material is the threshold activeinhibitor. Such materials are effective as scale inhibitors in amountsconsiderably less than that stoichiometrically required, and this amountis termed the threshold amount. Inorganic polyphosphates have long beenused as such threshold active inhibitors in the traditional scaleinhibition art. For examples of such materials, see Fink--U.S. Pat. No.2,358,222; Hatch--U.S. Pat. No. 2,539,305; and Ralston U.S. Pat. No.3,434,969. Certain water soluble polymers, including groups derived fromacrylamide and acrylic acid have been used to condition water containingscale-forming calcium carbonate. For example, see U.S. Pat. Nos.2,783,200; 3,514,476; 2,980,610; 3,285,886; 3,463,730; 3,518,204;3,928,196; 3,965,027; and 4,936,987. In particular, there has beenemployed anionic polyelectrolytes such as polyacrylates, polymaleicanhydrides, copolymers of acrylates and sulfonates, and polymers ofsulfonated styrenes. See, for example, U.S. Pat. Nos. 4,640,793;4,650,591; 4,671,888 and 4,072,607. However, when used as thresholdalkaline earth metal scale inhibitors, large dosages of these polymersare required, which in turn increases operating costs.

U.S. Pat. No. 4,457,847 discloses antiscalant use of a water-solublesequestrant anionic vinyl polymer containing at least 30% by weight ofcarboxylate functionality, and having a molecular weight within therange of 500-50,000.

While various polycarboxylates, including polyacrylic acid, have beenused as scale inhibiting agents in the traditional scale inhibition art,as described above, no similar use has been made of polycationic agents,apparently because of the difference in electronic charge and theconventional theories of the mechanisms of action for polymericthreshold inhibitors and dispersants. However, Goodman U.S. Pat. Nos.4,164,521; 4,166,040; and 4,205,143 disclose compositions for inhibitingmagnesium scale in desalination evaporators comprising mixtures of apolyanionic such as acrylic acid and a polycationic includingpoly(dimethyldiallylammonium chloride); but, there is no suggestion ofthe use of such compositions to control black liquor scale inpapermaking, or of the preferred embodiments of the present invention,which have particularly advantageous properties in that regard.

As already stated, none of the scale inhibition agents used in thetraditional scale inhibition art and described above have met with anynotable success when used to prevent scale formation in black liquorprocessing. These agents have shown little or no effect in overcomingthe specific problem of scale deposit formation under the stringentconditions which characterize black liquor processing in paper making.Some specific applicability to this area has been asserted for certaincompositions, but little success in actual practice has been realized.For example, U.S. Pat. No. 4,357,207 discloses the use of cationicpolymers having a molecular weight of from 2,000 to 50,000 to inhibitand disperse deposits on black liquor evaporators. U.S. Pat. No.3,516,910 discloses the use of polymethacrylates and relatedpolyalkylacrylates having a molecular weight from 1,000 to 200,000 toinhibit deposit formation on black liquor evaporators. U.S. Pat. No.3,289,734 discloses a method of inhibiting deposit formation on blackliquor evaporators by treatment with a styrene-maleic anhydridecopolymer having a molecular weight of less than 10,000.

It is probable that the anionic/cationic polymer mixture of the presentinvention forms a water-soluble nonstoichiometric polyelectrolytecomplex. Such materials are known and have been investigated in thepast, but never with any suggestion that they would be useful in scaleinhibition in a normal setting, let alone under the stringent conditionswhich characterize black liquor processing in paper making. See, e.g.,Kabanov and Zezin, Makromol. Chem. Suppl. 6, 259-276 (1984).

SUMMARY OF THE INVENTION

The present invention relates to a composition useful as a depositcontrol agent to control the formation, depositon and adherency of scaleimparting compounds in an aqueous system involving the processing ofblack liquor during paper making, comprising an anionic/cationic polymermixture of:

(a) a polyanion selected from polyacrylic acid, polymethacrylic acid,and polymaleic anhydride, each optionally copolymerized with each other,or optionally copolymerized with acrylamide up to a molar unit ratio of1:1, provided that the total polyanionic weight average molecular weightis from about 1 to 5 thousand, and

(b) the polycation poly(dimethyldiallylammonium chloride) having aweight average molecular weight of from about 25 thousand to about 1million,

wherein the weight ratio of polyanion (a) to polycation (b) is fromabout 1:2 to about 1:8.

The present invention particularly relates to the composition describedabove for controlling the deposition of sodium sulfate, sodiumcarbonate, organic matter, and calcium carbonate on the surfaces ofblack liquor evaporators and heaters associated with Kamyr digesters,wherein the composition is a mixture of polyacrylic acid having a weightaverage molecular weight of about 2 thousand, andpoly(dimethyldiallylammonium chloride) having a weight average molecularweight of about 1 million, and the weight ration of polyanion topolycation is about 1:2.

The present invention also relates to a method of inhibiting theformation, deposition and adherency of scale-forming salts in an aqueoussystem involving the processing of black liquor during paper making,comprising the step of adding to said system an amount sufficient toestablish a concentration of from 1 to 100 mg/L of an anionic/cationicpolymer mixture of:

(a) a polyanion selected from polyacrylic acid, polymethacrylic acid,and polymaleic anhydride, each optionally copolymerized with each other,or optionally copolymerized with acrylamide up to a molar unit ratio of1:1, provided that the total polyanionic weight average molecular weightis from about 1 to 5 thousand, and

(b) the polycation poly(dimethyldiallylammonium chloride) having aweight average molecular weight of from about 25 thousand to about 1million,

wherein the weight ratio of polyanion (a) to polycation (b) is fromabout 1:2 to about 1:8.

The present invention particularly relates to the method of treatmentdescribed above for controlling the deposition of sodium sulfate, sodiumcarbonate, organic matter, and calcium carbonate on the surfaces ofblack liquor evaporators and heaters associated with Kamyr digesters;wherein the composition is a mixture of polyacrylic acid having a weightaverage molecular weight of about 2 thousand, andpoly(dimethyldiallylammonium chloride) having a weight average molecularweight of about 1 million, and the weight ratio of polyanion topolycation is about 1:2; and wherein the concentration of thecomposition is from 10 to 50 mg/L.

DETAILED DESCRIPTION OF THE INVENTION

The composition of the present invention useful as a deposit controlagent to control the formation, deposition and adherency of scaleimparting compounds in an aqueous system involving the processing ofblack liquor during paper making, comprises an anionic/cationic polymermixture of:

(a) a polyanion selected from polyacrylic acid, polymethacrylic acid,and polymaleic anhydride, each optionally copolymerized with each other,or optionally copolymerized with acrylamide up to a molar unit ratio of1:1, provided that the total polyanionic weight average molecular weightis from about 1 to 5 thousand, and

(b) the polycation poly(dimethyldiallylammonium chloride) having aweight average molecular weight of from about 25 thousand to about 1million.

wherein the weight ratio of polyanion (a) to polycation (b) is fromabout 1:2 to about 1:8.

The polyanion is selected from polyacrylic acid, polymethacrylic acid,and polymaleic anhydride, whose compositions may be represented instructural form by the following general formulas, respectively:##STR1## where "x", "y" and "z" are numbers representingrepeatingmonomer units sufficient to obtain the desired weight average molecularweight.

It will be understood that the polyanions shown above are represented aspolycarboxylic acids, but that the alkali metal and ammonium saltsthereof may be employed as well.

In the most preferred embodiments of the present invention, thepolyanion component of the anionic/cationic polymer mixture is ahomopolymer, e.g., polyacrylic acid. However, in less preferredembodiments, it is also possible to use copolymers and even terpolymersas the polyanion component which comprise the members of the group fromwhich the polyanion is selected. For example, a copolymer of acrylicacid and methacrylic acid, or acrylic acid and maleic anhydride may beemployed. The terpolymer would comprise acrylic acid, methacrylic acid,and maleic anhydride.

It is also optional to copolymerize a homopolymer of acrylic acid ormaleic anhydride, for example, with acrylamide, or a copolymer ofmethacrylic acid and maleic anhydride, for example, with acrylamide.Polyacrylamide is a nonionic polymer material and, as such, is notviewed as disturbing the balance of anionic and cationic charges whichare theorized to be essential to the functioning of the anionic/cationicpolymer mixtures of the present invention. The polyacrylamide portiondoes, however, add molecular weight to the polyanion component and is,consequently, viewed as essentially diluting the extent of anioniccharge, or charge density. For that reason, it is a limitation of thepresent invention that the molar unit ratio of optional acrylamidemonomer to the total of anionic monomers (if more than one is used), canbe no more than 1:1, i.e., up to 1:1. For example, if anacrylamide/acrylic acid copolymer is used in a 1:1 ratio, since themolecular weights are nearly equal, the requirement that the anioniccomponent molecular weight be between 1 and 5 thousand would mean thatthe overall weight average molecular weight of thepoly(acrylamide/acrylic acid) should be between 2 and 10 thousand.

However, in the most preferred embodiments of the present invention,polyacrylic acid is used as a homopolymer and is not copolymerized withacrylamide. For the polyacrylic acid component, it is also mostpreferred to use the homopolymer having a molecular weight of about 2thousand, as a weight average molecular weight, which would beunderstood to be an approximately average number and subject toreasonable variation.

As noted, it is essential that the weight average molecular weight ofthe polyanion component of the anionic/cationic polymer mixture be inthe range of from about 1 to 5 thousand, and in the most preferredembodiment, it is about 2 thousand. It has been found that when themolecular weight exceeds about 5 thousand, that a considerable reductionin scale inhibiting properties ensues. By contrast, the weight averagemolecular weight of the poly(dimethyldiallylammonium chloride) componentof the anionic/cationic polymer mixture of the present invention must beat least about 25 thousand, but may go as high as about 1 million oreven higher. In the most preferred embodiments of the present invention,the weight average molecular weight of the polycationic component isabout 1 million. With very high molecular weights, however, scaleinhibiting properties decline without any offsetting advantage.

The ratio of the anionic and cationic components of the polymer mixturecompositions is another critical aspect of the present invention. Thatratio is based on overall weight of the polymer components (rather thanbeing a molar ratio) and can range from about 1:2 to about 1:8, but willpreferably be in the range of from 1:2 to 1:4, and is most preferablyabout 1:2. Thus, the cationic component of the polymer mixture, i.e.,the poly(dimethyldiallylammonium chloride), will usually be thepredominant component by weight.

The compositions of the present invention are anionic/cationic polymermixtures or blends, as distinct from random copolymers of the ordinarytype. It is conjectured, however, that the mixture may form segmentedblock copolymers, as detailed further below. In any event, a significantdifference in activity results. Thus, it has been discovered that whenanionic polyacrylic acid and cationic poly(dimethyldiallylammoniumchloride) are combined in a copolymer at a weight ratio within the rangeof the polymer mixtures of the present invention, that no effect onadherency of the scale is achieved, in contrast to the results achievedwith the compositions and methods of the present invention.

It is theorized that there is an interaction between the polyanioncomponent of the polymer mixture and the Na₂ SO₄, Na₂ CO₃, CaCO₃ andother scale crystallites which constitute the nucleus of each scaleparticle potentially deposited, such that there is a thresholdinhibition of formation of a particle of scale of sufficient size to bedeposited; while the polycation poly(dimethyldiallylammonium chloride)component of the polymer mixture neutralizes the charges on the scalecrystallites, thereby flocculating them and preventing them fromproducing adherent scale deposits.

The anionic and cationic polymer mixtures of the present invention, inthe ranges of molecular weights and weight ratios described above, areblends of the two homopolymers. The blending is achieved with the use ofconventional apparatus well known in the art, and no particulardifficulty will be encountered inmaking the polymer blends whichcomprise the compositions of the present invention. Insoluble polysaltcomplexes may be formed, but these can be easily avoided by adjustingthe pH so that the polyacrylic acid, normally anionic, is present as theun-ionized acid form.

The anionic/cationic polymer mixture of the present invention may beprepared either by adding the anionic homopolymer to the cationichomopolymer with mechanical stirring, or by reversing that order ofaddition. Because of the difference in molecular weights and weightproportions of the two components in the overall mixture, however, thereis a clear difference in the way these two processes of preparation bymixing proceed on a molecular level. Where the higher molecular weightand higher weight proportion polycationic component is added to thepolyanionic component, it is expected that there will be a tendencyinitially to form water-insoluble polysalt complexes, because at thepoint of addition the two components will tend to be present instoichiometric amounts. However, as further amounts of the predominantpolycationic component are added, any water-insoluble polysalt complexeswhich may have formed will be considered to have dissolved, i.e., theequilibrium with the water-soluble non-stoichiometric polyelectrolytecomplexes being formed, will have shifted. Where the order of additionis reversed, i.e., where the polyanionic component is added to thepolycationic component, there will be a tendency to more easily form thepolyelectrolyte mixture of the present invention, since the polyanioniccomponent has the smallermolecular weight is is present in a smallerweight proportion.

It is theorized that the polyanionic and polycationic components of thepolymer mixtures of the present invention may form what are, in effect,segmented block copolymers in which the polyanionic component allignsitself in discrete segments with the longer chains of the polycationiccomponent by ionic bonding. However, not only are these polyanionicsegments capable of movement within the same polycationic chain or to adifferent polycationic chain, but it is also clear that the segmentednature of the polyelectrolyte complex formed will result in hydrophilicand hydrophobic regions which can be both intramolecular andintermolecular, i.e., they can result in conformational changes within asingle anionic/cationic complex, or result in agglomeration of suchcomplexes by mutual attraction of the hydrophobic regions. One or more,or even all of these effects may play a critical role in enabling theanionic/cationic polymer mixtures of the present invention to inhibit,at threshold levels of concentration, the formation, deposition andadherency of scale-forming salts under the stringent conditions of blackliquor processing in paper making, where virtually all conventionalagents tried heretofore have failed.

When any of the anionic and cationic polymer mixture compositions of thepresent invention are used to inhibit the precipitation, deposition, andadherence of scale-forming salts in an aqueous system involving blackliquor processing during papermaking, they can be effectively employedfor that purpose when added in amounts sufficient to establish aconcentration in said black liquor processing aqueous system of from 1to 100 mg/L. Preferably, the amount added will be sufficient toestablish a concentration of from 5 to 75 mg/L, and most preferably, theamount added will be sufficient to establish a concentration of from 10to 50 mg/L of the compound. It is understood, however, that manyfactors, of the type which have been explained in detail with regard tothe background to the present invention, will determine the actualamount of the anionic and cationic polymer mixture compositions of thepresent invention which will be added to said black liquor processingaqueous system in order to achieve the maximum amount of inhibition ofalkaline earth metal, especially calcium carbonate scale formation,deposition and adherence in said aqueous system. The calculation ofthose amounts will be well within the skill of the artisan in thisfield.

The phrases "inhibiting the precipitation" and "inhibiting the formationand deposition" are meant to include threshold inhibition, dispersion,solubilization, or particle size reduction. The phrases "inhibiting theadherence" and "increasing the non-adherence", are meant to define theformation of a scale deposit which is easily removed, e.g., by simplerinsing, i.e., a scale deposit which is not so firmly bonded to thesurface to which it is attached that it cannot be removed by simpleagitation means as opposed to harsh mechanical or chemical treatment.

The phrase "scale-forming salts" is meant to include any of thescale-forming salts, including, but not limited to, sodium sulfate,sodium carbonate, calcium carbonate, calcium sulfate, calcium phosphate,calcium phosphonate (including calcium hydroxyethylidene diphosphonicacid), calcium oxalate, calcium fluoride, barium sulfate and magnesiumsalts.

The phrase "black liquor processing aqueous system" means any of thosepoints or areas in the paper making process and its attendant equipmentin which black liquor is processed in such a way that it comes incontact with heat exchange surfaces, usually of metal, or in which itotherwise has a tendency to form adherent scale deposits. Thus, theseinclude, e.g., black liquor evaporators and black liquor heaters andflash evaporators used in conjunction with Kamyr and other similar woodchip digesters. These examples are not meant to be limiting, however,since there are other points and areas in such Kamyr and other digesterswhere the black liquor tends to leave an adherent scale deposit.

The manner of addition of any particular anionic and cationic polymermixture composition of the present invention, to a black liquorprocessing aqueous system will also be straightforward to a person ofordinary skill in this art. It may be added in liquid blend form bymechanical dispensers of known design. It may also be added in dilutedliquid form. The anionic and cationic polymer mixture composition mayalso be combined with other chemical treatment agents for dispensing tothe black liquor processing aqueous system; and these in combination maybe dispensed in liquid form.

The anionic/cationic polymer mixtures of the present invention may beadded at a number of different points in the paper making process so asto become dispersed in the black liquor processing aqueous system.Considerations of convenience and efficiency will usually govern thischoice. The mixtures are effective when added to the steam lines or tothe vapor phase above any particular black liquor processing unit suchas a heater or evaporator. The mixtures may also be added directly tothe black liquor in such units, or to any of the lines which transportthe black liquor from place to place. The mixtures may also be added tothe wash or white liquor, which is essentially dilute black liquor priorto its concentration to form what is traditionally known as blackliquor. It is also possible to add the mixtures of the present inventionat two or more of the points in the paper making process describedabove. As a general matter, however, it is preferred to add the mixturesof the present invention at a single point early in the paper makingprocess where black liquor processing aqusous systems become involved,so that the treatment composition will be available during anyprocessing of the black liquor by being carried through with the blackliquor.

The compositions and methods of treatment of the present inventiondescribed above are effective in reducing deposit formation occurringupon a variety of metal surfaces. Protection is afforded equipment madeof ferrous metals such as iron and steel or non-ferrous metals such as,e.g., brass, which are or may be exposed to black liquor duringevaporation, condensation, transportation, concentration, or otherchemical and physical processing of black liquors.

EXAMPLES OF PREFERRED EMBODIMENTS Example 1 Experimental Autoclave Study

The study was done in an autoclave made of Inconel. A U shaped tube madeof No. 316 stainless steel, suspended from the lid of the autoclave wasused as the test specimen for measuring deposit. One liter of blackliquor containing an additional 170 mg of calcium ions was charged intothe autoclave. The control experiment contained no inhibitor, while thetreated experiments contained different levels of treatment. A clean,polished (with 320 grit sand paper), and preweighed specimen tube wasput in place. A long shaft with two stirring propellers at differentheights was attached to the inside of the lid and this shaft at theoutside of the lid was equipped with a pully. The pully was attached bya belt to a motor by means of which stirring of the liquor wasaccomplished. The autoclave was inserted into a jacket containing anelectrical heater. A thermocoupler was inserted into a pocket attachedto the inside of the autoclave lid. The signal from the thermocouplerwas fed to the voltage controller of the heater to maintain a pre-setconstant temperature inside the autoclave throughout the experiment.Theautoclave was also equipped with a pressure guage to measure thepressure inside the autoclave.

The liquor containing additional calcium with or without the treatmentwas heated for 6 days at a constant temperature of 150° C. and ˜180 PSIpressure, after which it was cooled to 42° C. before opening of theautoclave. To clean the superficial liquor from the specimen tube, itwas dipped into distilled water for 2-3 seconds while still attached tothe lid. The specimen tube was detached from the lid, dried at 105° C.and weighed.

The composition of the black liquor containing additional calcium whichwas utilized in the above study was as follows:

                  TABLE 1                                                         ______________________________________                                        Characterization of Black Liquor                                                                 Amount                                                     Contents           ppm                                                        ______________________________________                                        pH - 13.8                                                                     Total Solids (105° C.)                                                                    450,000                                                    Dissolved Solids   224,000                                                    Suspended Solids   2,300                                                      Total Organics     60,000                                                     Total Ca           420                                                        Dissolved Ca       330                                                        Dissolved Hardness (CaCO.sub.3)                                                                  1025                                                       Total Mg           75                                                         Total Na           72,500                                                     Total K            15,500                                                     Total SO.sub.4     8,000                                                      Total Cu           05                                                         Total Fe           10                                                         Total Mn           25                                                         Total Al           20                                                         Total Zn           05                                                         Total Ni           05                                                         Total CrO.sub.4    05                                                         ______________________________________                                    

The results obtained from the above autoclave study, in which thetreatment composition was an embodiment of the present invention, beinga mixture of 1:4 of polyacrylic acid (AA) of about 3000 weight averagemolecular weight, and poly(dimethyldiallyl ammonium chloride) (DMDAAC)of about 40-70 thousand weight average molecular weight, are illustratedin the following table of values:

                  TABLE 2                                                         ______________________________________                                        Treatment      Dosage (ppm)                                                                              Deposit (g)                                        ______________________________________                                        Blank          --          8.69                                                 "            --          7.14                                               1:4 AA/DMDAAC  30          0.13                                               "   "          15          0.71                                               "   "            7.5       2.28                                               ______________________________________                                    

Example 2

Following the procedures described above in Example 1, and using thepreferred embodiment of the present invention in which the polyanion ispolyacrylic acid of weight average molecular weight about 2 thousand,and the polycation is poly(dimethyldiallylammonium chloride) of weightaverage molecular weight about 1 million, and the weight ratio of thefirst to the second is about 1:2, similar results are obtained showingthe composition to be an effective inhibitor of black liquor scaledeposition.

What is claimed is:
 1. A composition useful as a deposit control agentto control the formation, deposition and adherency of scale impartingcompounds in an aqueous system involving the processing of black liquorduring paper making, comprising an anionic/cationic polymer mixtureof:(a) a polyanion selected from polyacrylic acid, having a weightaverage molecular weight of about 2 thousand, and (b) the polycationpoly(dimethyldiallylammonium chloride) having a weight average molecularweight of 1 million,wherein the weight ratio of polyanion (a) topolycation (b) is about 1:2.